1. Technical Field
This invention relates to a numerical control apparatus in which an operation for returning the spindle of a spindle motor to a reference point is improved. More particularly, this invention relates to a numerical control apparatus adapted in such a manner that a spindle is returned to a reference point merely by executing a single-step reference-point return command.
2. Background Art
Generally, in a numerically controlled machine tool, such as a machining center or a numerically controlled lathe, an orientation function is required for stopping a spindle at a predetermined machine reference point as when automatically changing tools. Methods of commanding return to a reference point for the purpose of implementing an orientation function of this kind include a method in which a servomotor or the like is provided separately of a spindle motor, and a method in which a spindle amplifier is provided with a spindle reference-point restoration function and a command necessary for orientation is supplied by an NC apparatus.
However, the method in which a servomotor is separately provided has economical disadvantages. Specifically, a more complicated mechanical structure cannot be avoided, and a motor for returning the spindle to the reference point must be installed. In the arrangement wherein a spindle amplifier is internally provided with a spindle reference-point return function, the signal system for commands from the NC apparatus and for reference-point setting commands is complicated, and control for positioning the spindle cannot be carried out with a single-step reference-point return command. Accordingly, in order to run the NC apparatus, the command program steps must be modified depending upon the type of machine tool to be numerically controlled.